1. Field of the Invention
This invention relates to radiography apparatus adapted to provide improved contrast sensitivity while permitting reduced radiation exposure. More specifically, this invention relates to radiography apparatus employing a self-scanning array of photodiodes.
2. Description of the Prior Art
In the use of radiation, such as x-rays, gamma rays and nuclear particles, in view of the potential hazardous effects, means have been provided in order to minimize the morbidity and mortality of radiation exposure. The hazards which result from excessive exposure to radiation exist not only where a patient is being subjected to radiation but also with respect to personnel in the surrounding area. One known means for controlling or reforming an x-ray beam before it reaches an object and also minimizing patient exposure to the radiation is disclosed in U.S. Pat. No. 3,973,127 which relates to x-ray tomography. See also U.S. Pat. Nos. 3,947,689; 3,829,701; 3,934,151 and 3,767,931.
In respect of the desire to reduce the radiation exposure, conflicting objectives are encountered. In general, the clarity of the image and ability to perceive contrast so as to reveal the presence of small departures from the desired condition, e.g. a small tumor, requires meaningful radiation exposure. Mere reduction in radiation exposure tends to contribute to deterioration of perception of contrast and detail in the image.
U.S. Pat. No. 3,866,047 discloses the use of a plurality of pencil beams of radiation which are collimated and converted into light which impinges upon one or more photomultipliers. A computer processes the electrical signals emitted by the photomultipliers. Among the problems with this approach are the severe limitations on resolution imposed by the relatively large size of the photomultipliers and the cost of the same. In addition, an inherent difficulty with photomultipliers is their "after-glow" or noise subsequent to exposure to very high levels of light intensity. This memory effect serves to interfere with the efficiency of the system. In addition, this approach is not readily compatible with existing diagnostic x-ray equipment. Further, the use of a pencil beam increases the exposure time and the heat loading of the x-ray tube. In addition, the support equipment, such as independent amplifiers required for each photomultiplier, further increases the cost and physical mass of the system. U.S. Pat. No. 4,010,370 discloses apparatus for computerized tomography wherein a plurality of individual photomultipliers of photodiodes are used with collimators and scintillation crystals, in order to convert x-ray into light and ultimately into an electrical signal containing the image data. The cumbersome use of individual detector means and the associated processing electronics perpetuates a number of shortcomings of U.S. Pat. No. 3,866,047. See also U.S. Pat. No. 4,010,371. U.S. Pat. No. 4,029,964 discloses a scintillation camera adapted for use in nuclear medicine to receive gamma rays resulting from radioactive disintegrations of the radioisotope administered to a patient. The equipment employs a plurality of photodetectors in the form of photomultipliers tubes which are optically coupled as by light pipes to a scintillation detector.
There remains, therefore, a need for a diagnostic radiography system which is adapted for use with reduced levels of radiation while providing improved images or image data of improved contrast sensitivity and detail. There is a further need for such equipment which is compatible with existing radiography equipment and economical to manufacture and use.